1. Field of the Invention
The present invention relates to a furnace bottom structure of a direct current electric furnace which melts and refines metals by arcs produced by a direct current.
2. Description of the Related Art
Conventionally, steel making arc furnaces are of alternating current type. In recent years, however, thyristors and diodes serving as large-capacity AC/DC converters are available and, for this reason, direct current electric furnaces are attracting attention.
In producing a steel by a direct current electric furnace, scraps are loaded in the furnace and then electric power is supplied to the electrodes of the furnace thereby melting the scrap and refining the melt. In order to promote melting and refining, oxygen gas is supplied into the furnace through a lance which is inserted in a charging hole. Feeding of a lance into an electric furnace, however, requires a plurality of workers who are obliged to sustain heavy and laborious work at high air temperature. This is quite undesirable from the view point of safety.
Apparatus has been proposed which can feed a lance by means of a truck. Such an apparatus requires a large installation space which restricts the working space in front of the furnace.
Moreover, oxygen blowing through a lance alone cannot provide sufficient melting effect and does not provide uniform composition of the melt due to insufficient stirring effect.
As an improvement in the above-described art which makes use of a lance, an electric furnace has been proposed in Japanese Patent Laid-Open No. 57-60011 in which a plurality of tuyeres for blowing an oxidizing gas or an inert gas are disposed under the upper electlode on the furnace bottom along a circle concentric to the electrode. This improved electric furnace, however, suffers from a problem in that, during the supply of electric power to the upper electrode, electric current flows through the furnace shell to generate sparks between MgO-C type tuyere bricks and the tuyere so that the tuyere can become molten allowing leakage of gas. Consequently, the operation of the furnace becomes stable and dangerous. In order to overcome this problem, an electric furnace has been proposed in Japanese Patent Laid-Open No. 57-188980 in which the surface of an outer tube of a double tuyere is coated with an electrically insulating material while the double tuyere itself is grounded, thereby preventing generation of arc. This art, however, is intended to prevent generation of an arc only at the gap between the furnace bottom gas blowing tuyere and the tuyere bricks around the tuyere.
All of this known arts is intended for overcoming problems encountered with the alternating current electric furnace, and no proposal has been made for elimination of the same problem in direct current furnaces.